feat(history): improve commit hover branch labels and titles

The history panel now generates more informative hover text for commits.
It prefers direct local branch refs when available, otherwise it derives
a concise list from containing branch labels and summarizes overflow.

- Add helpers to compute hover branch labels and title text
- Update the graph row hover rendering to use the new helpers

src/lib/components/HistoryPanel.svelte

@@ -207,6 +207,24 @@
     return labels.filter(branchIsVisible);
   }
 
+  function commitHoverBranchLabels(commit: GitCommit, row: GraphRow | undefined): string[] {
+    const directBranches = localBranchRefs(commit);
+    if (directBranches.length > 0) return directBranches;
+
+    const containingBranches = row?.branchLabels ?? [];
+    if (containingBranches.length <= 3) return containingBranches;
+    return [...containingBranches.slice(0, 3), `+${containingBranches.length - 3} more`];
+  }
+
+  function commitHoverTitle(commit: GitCommit, row: GraphRow | undefined): string {
+    const directBranches = localBranchRefs(commit);
+    if (directBranches.length > 0) return `Branches: ${directBranches.join(", ")}`;
+
+    const containingBranches = row?.branchLabels ?? [];
+    if (containingBranches.length === 0) return commit.short_hash;
+    return `Branches containing this commit: ${containingBranches.join(", ")}`;
+  }
+
   function segmentIsVisible(segment: GraphSegment): boolean {
     return segment.branches.length === 0 || segment.branches.some(branchIsVisible);
   }
@@ -483,7 +501,7 @@
       {#each visibleCommitEntries as entry, rowIndex (entry.commit.hash)}
         {@const item = entry.commit}
         {@const row = graphRows[rowIndex]}
-        {@const hoverBranchRefs = visibleBranchLabels(row?.branchLabels ?? [])}
+        {@const hoverBranchRefs = commitHoverBranchLabels(item, row)}
         {@const otherRefs = visibleRefs(item)}
         <article
           class="commit-row graph-row"
@@ -518,7 +536,7 @@
                 class:merge={item.parents.length > 1}
                 class:tip={item.refs.length > 0}
                 class:hidden-branch={!rowGraphIsVisible(row)}
-                title={hoverBranchRefs.length > 0 ? `Contained in: ${hoverBranchRefs.join(", ")}` : item.short_hash}
+                title={commitHoverTitle(item, row)}
                 style={`left:${graphColX(row.dotCol)}px; --dot-color:${row.dotColor}`}
               ></span>
               {#if hoverBranchRefs.length > 0}